1. Field of the Invention
The present invention relates to an electroless plating method for the fabrication of targets for vacuum sputtering of nickel-phosphorous alloys and other materials.
2. Description of Related Art
Sputtering (also known as vacuum sputtering or physical vapor deposition) is uniquely suited for controlled and reproducible deposition of thin films on an atom layer-by-atom layer basis. Sputtering is the preferred method for adding the essential magnetic layer to memory disks used in computer disk drives. Sputtering is used in the fabrication of heads which write and read magnetic memories and is also used extensively in semiconductor chip fabrication. In addition to depositing thin layers of metals, sputtering is also a means for adding thin layers of insulating materials such as glass and aluminum oxide.
Details of the mechanism of sputtering may be found in texts on plasma physics such as “Glow Discharge Processes” by B. Chapman (John Wiley and Sons, NY 1980, ISBN 0-471-07828-X, Chapter 6, pages 177-194). Basically, atoms are knocked loose from a source target when argon ions energetically collide with the source target. A small amount of inert argon gas added to a vacuum becomes ionized in an electrical field applied between two electrodes. Positively charged ionized argon ions are accelerated by the voltage towards the negative target electrode. When the speeding argon ions collide with the target, the energy of the collision knocks target atoms loose which are then free to move through the vacuum to build up layers on the opposite electrode surface.
The typical components of a sputtering system include a vacuum chamber, vacuum pumps, source targets, inert gas control, and high voltage power supply. Targets are designed to be readily removed and replaced when sufficient material is eroded away as sputtering proceeds. Target design includes provision for cooling and electrical isolation of high voltage in the vacuum chamber. Customarily, the source material is in the form of a thick layer, typically 6 mm, bonded to a reusable copper backing plate using low-melting indium or indium alloy solder.
Sputter targets of metals and metal alloys are conventionally fabricated by two major methods:                1) metal or metal alloy melting, casting, and subsequent shaping and machining;        2) powder metallurgical methods such as hot-pressing or sintering of pure metal powder or a mixture of powders of desired alloy components.        
As recent examples of alloy melting, Lam et al. (U.S. Pat. No. 6,342,114) describe vacuum melting of nickel-vanadium sputter targets. Ivanvov (U.S. Pat. No. 6,423,196) describes casting ingots from a molten mixture of nickel and silicon whereas Yi et al. (U.S. Pat. No. 6,713,391) describe forming silicon-nickel (and other silicon alloy) targets by powder metallurgical methods.
Powder metallurgy methods require presses of substantial size coupled with dies that sustain high temperature heating and which may require simultaneous application of vacuum. Cycles for degassing of blended powder compacts, uniaxial compression, multiple stages of sintering and hot pressing followed by slow cooling of intermetallic compound targets are described by Yi et al. (U.S. Pat. No. 6,713,391). Lo et al. (U.S. Pat. No. 6,328,927) describe hot-isostatic pressing (H.I.P.) of tungsten powder to form a sputter target. Powders are generally required to be of closely controlled, finely divided particle sizes to facilitate sintering. Han et al. (U.S. Pat. No. 6,676,728 and U.S. Pat. No. 6,589,311) describe H.I.P. of plasma-sprayed powders of various high-melting metals to form sputter targets.
In a variant of method 1, alloys may also be sputtered from a target comprised of a mechanically assembled mosaic array of pure solid constituent metals inlaid in a matrix, as described by Kailasam et al. U.S. Pat. No. 6,709,557
The powder metallurgical techniques of hot pressing or hot isostatic pressing (H.I.P.) have been used for manufacturing nickel-phosphorous (Ni—P) targets. A major application of these Ni—P targets has been as a source for thin sputtered layers on glass substrates to provide an optically opaque layer to facilitate laser texturing of a head landing zone. A new application of sputtered Ni—P in the disk industry is for adding a seed layer (Nanis U.S. Pat. No. 5,405,646) to nucleate electroless nickel plating on high strength metal substrates which are alternatives to glass and conventionally used aluminum-magnesium alloys.
The starting material for nickel-phosphorous powder metallurgy may be obtained by aqueous chemical precipitation methods as described by Kageyama et al. (U.S. Pat. No. 6,627,118). Targets of Ni—P made in this manner are sintered so as to produce a narrow range of porosity. Porous targets are less desirable than solid targets since they require lengthy pump down times to remove entrained gases and to achieve good vacuum levels in the sputtering chamber. Also, porous target material does not conduct heat as well as solid metal targets, thereby limiting target power levels and thus sputtering rates.
Melting and casting methods are not presently used for making fully dense Ni—P targets because highly reactive elemental phosphorous is difficult to add directly to molten nickel. The disclosed technology describes nucleated electroless plating as a novel and simple method different from casting or powder methods for fabricating solid, 100 percent dense nickel-phosphorous and similar alloy sputter targets.